Diapsida

Aside from having two openings in the temporal region of the skull, diapsids typically have hind limbs that are longer than the forelimbs. The oldest known diapsid was a small, lizardlike animal with a body length (minus the long tail) of about 8 in (20.3 cm). This slender animal, named Petrola-cosaurus, was collected from the late Pennsylvanian (ca. 303-290 mya) of Kansas.

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(barrier to diffusion)

Pre-amnionic

Amnionic Egg fibrous shell fibrous shell

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Extraembryonic membranes:

allantois yolk sac amnion chorion fibrous shell

JM©2002

Evolution of the amniotoic egg. (Illustration by Jacqueline Mahannah) Grzimek's Animal Life Encyclopedia

Two distinct clades, the Lepidosauria and the Archosauria, branched off early from the diapsid trunk. These two groups are characterized by their contrasting patterns in locomotion and posture. The lepidosaurs retained the sprawling posture and laterally directed movement of the limbs found in primitive tetrapods. Lateral undulation of the vertebral column was also an important method of locomotion for most lepi-dosaurs, reaching its highest degree of development in snakes. Loosely separated skull bones, which allowed prey to be swallowed whole, was another important lepidosaur feature. On the other hand, archosaurs had limited or absent lateral undulation in the vertebral column, and the limbs were brought into a position more directly under the trunk. These modifications reached their highest degree of development in the dinosaurs and pterosaurs.

Representative species of the three groups of the Lepi-dosauria addressed here, the sphenodontids, lizards, and snakes, are presently alive. Turning to the sphenodontids, the tuatara (Sphenodon) of New Zealand are the only living members of this once large group. A newspaper article dating to the 1940s explained that the tuatara looked something like a lizard but really was a "living fossil." There are several differences between the sphenodontids and lizards, which split off from each other in the Triassic, possibly early in the epoch. In the sphenodontids, the jaw muscles are massive, which allows them to have a stronger but slower bite than lizards. Sphenodontid teeth are fused to the jaw so firmly (acrodont condition) that the jaw has a sawtooth appearance, as if the teeth are merely serrations of the bone itself. Jaw muscles are less massive in lizards and are located farther back in the mouth, producing a weaker but faster bite. Most lizards have teeth somewhat loosely attached to the inside of the jaw (pleu-rodont condition), and these teeth are replaced frequently in most species.

Sphenodontids were the dominant lepidosaurs of the Jurassic (206-144 mya), but they sharply declined in the Cretaceous as the lizards began to diversify broadly. Only the two species of New Zealand tuatara have survived to the present, others having died out at the end of the Cretaceous.

Fossil of a mosasaur—a giant sea lizard from the Cretaceous period. (Photo by R. T. Nowitz Photos/Photo Researchers, Inc. Reproduced by permission.)

The tuatara are active at much lower temperatures than most lizards, and the eggs have a gestation period of about nine months before being laid. The incubation period for the eggs is about 15 months, the longest of any known living reptile. Growth in the young is slow, and the animals do not reach sexual maturity until they are about 20 years old. Slow growth then continues until the animal is 50 to 60 years of age. Rather than having hemipenes (double penis) like lizards and snakes, male tuatara transfer sperm to the female by an extension of the gut called the cloaca. Whereas most lizards seem to look right through a person, tuatara have a direct gaze, with big brown eyes that seem more mammalian than reptilian.

Lizards and snakes are considered to be a single clade, the Squamata (scaled reptiles). Both lizards and snakes have legless forms with a jaw structure that allows them to swallow prey whole. Snakes, however, have carried these tendencies to the extreme. The first lizards are represented by an animal known as Paliguana, from the late Permian of South Africa. The fact that lizards had a more effective jaw structure, better hearing, and improved locomotion probably allowed them to exploit the habitats occupied by other lizardlike tetrapods, such as the sphenodontids. Most modern lizards, with the exception of the Komodo dragon (a monitor lizard that can take down deer), have not achieved large size. But in the Cretaceous, ancestral monitor lizards evolved into the ecologically important mosasaur, marine lizards that grew to 30 ft (9.1 m) in length. One giant terrestrial lizard of the past was the monitor lizard Megalania (probably 20 ft or 6.1 m long), the top predator in the Pleistocene (1.8-0.1 mya) of Australia. Fossils of Megalania at first were thought to be those of dinosaurs, but Max K. Hecht of the American Museum of Natural History proved that they were, in fact, giant lizards.

Snakes originated much later in the fossil record than lizards, at some time during the Middle to Upper Cretaceous. The four fossils that bear most closely on the ancestry of snakes are Pachyrhachis, Podophis, Lapparentophis, and Dinilysia. The first three are from the Middle Cretaceous, but Dinilysia, the most complete and well-studied of the four, is from the late Cretaceous. The marine squamates Pachyrhachis and Podophis have been considered the most primitive snakes by some researchers, because the configuration of the skull bones resembles that of living snakes, but they have a well-developed hind-limb skeleton. The terrestrial snake Lapparentophis, often called the "oldest snake," is represented by vertebrae only, but they are certainly snake vertebrae, with all of the unique modifications found in generalized living snakes. Dinilysia has a skull that is a mosaic of lizard and primitive snake characters, but its vertebrae are clearly like those of a boa-like snake. Unfortunately, the whole picture of early snake evolution has been muddled by jargon-filled, convoluted arguments, the problem, as always, being the basic similarity of snakes and lizards.

Primitive snakes were dominant in the world until the Miocene, when modern snakes quickly replaced the less-advanced types. Three factors probably played a part: the return of warm and equable climates in the higher latitudes following the climatic deterioration in the Oligocene (ca. 33-23 mya), the striking spread of grassland habitats world wide, and the evolution of many rodent groups that could be exploited by snakes as food. The largest modern snakes include the boas of the New World and the pythons of the Old World. The giant python-like snake Wonambi lived in the Pleistocene of Australia, along with the giant lizard Megalania.

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